In recent years, in order to realize a smart house and M2M (Machine to Machine), there have been advanced research, development, and demonstration experiments of energy management based on remote supervision or remote control for home electric appliances and energy-related devices such as a solar power generation system and a storage battery. As a gateway device, between an external remote supervision • control server (hereinafter, referred to as just an external server) and in-home devices, a smart meter and a HEMS (Home Energy Management System) are mentioned. The external server sends an instruction to the smart meter or the HEMS via various communication media, and thereby, controls each of the devices.
As a device control performed by the external server, there are a restriction instruction regarding a generated power output of a solar power generation system and a specification of a charge and discharge amount of a storage battery. Each control instruction is constituted with an identifier of a control target, a cord indicating a generated power amount/charging or discharging of power, information indicating the content of an instruction to each code, and the like. The size of the control instruction is very small. For example, in ECHONET (registered trademark (omitted hereinafter); Energy Conservation and Homecare Network), a message format regarding storage battery control is specified. Data sizes in the message format are mostly 10 to 20 bytes.
As a communication medium for connecting between an external server and a consumer, it is considered to utilize a minimal band of around 100 bytes. Thus, an efficient use of such a limited communication band is required. At present, however, an external server transmits a plurality of kinds of control instructions to a consumer at a corresponding timing specified to each instruction. Thus, instructions having a small data size as described above flow on a narrow-band communication medium.
In the case where a large number of control instructions are communicated, there is a packet aggregation technology as one of technologies for improving data transmission efficiency. In this technology, a plurality of data are not transmitted as separate packets, but are transmitted in the form of, for example, a single packet into which the plurality of data are aggregated and contained.
According to this technology, it becomes possible to communalize a header area appended to a head portion of each of packets. Through this communalization of the header area, it is possible to efficiently utilize a bandwidth.
An example of such a technology is disclosed in PTL 1 and PTL 2.
In a technology disclosed in PTL 1, when an aggregated packet is generated, a size of the aggregated packet is dynamically changed. By this way, packet aggregation jitters and delay of a first aggregated packet can be reduced when voice data or the like is transmitted,
In a technology disclosed in PTL 2, a header portion is communalized when tunneling packets. That is, in the technology disclosed in PTL 2, when a large volume of data representing moving images or voice signals is transmitted, RTF (Real-time Transport Protocol) packets are encapsulated by using HTTP (HyperText Transfer Protocol). In this case, by communalizing header portions of the aggregated RTF packets, overhead due to the packet header is reduced.